A base station (also called an eNodeB) in a communication system such as Third Generation Partnership Project Long Term Evolution (3GPP LTE) system transmits downlink information to a user equipment (UE) through physical downlink control channel (PDCCH). Multiple PDCCHs can be transmitted in a subframe. Downlink control information (DCI) is carried by PDCCH, and is employed to transport at least downlink scheduling information or uplink scheduling information.
A DCI transports downlink or uplink scheduling information, or uplink power control commands for one RNTI (radio network temporary identifier). The RNTI is implicitly encoded in CRC (cyclic redundancy check), and each UE is assigned a unique RNTI, which can be UE unique identifier. There are two types of downlink control information in LTE system: common DCI and UE-specific DCI. The common DCI is for all UEs, and the UE-specific DCI is for a specific UE. With two types of downlink control information, each of them has its own search spaces such as common search spaces and UE-specific search spaces.
FIG. 1A is a schematic diagram illustrating a control region 11 in a subframe 10. Referring to FIG. 1A, the control region 11 is usually allocated from the beginning of the subframe 10. The control region 11 consists of a set of control channel elements (CCE) in a PDCCH, for example, 35 CCEs. Each of the CCEs (such as CCE 113) consists of 72 bits, and the CCE is the smallest unit of a control region as specified in 3GPP LTE release-8 standard. Usually, the size of the control region in a subframe is increased with the system bandwidth. In other words, the greater the system bandwidth is, the larger the control region will be. The control region 11 includes a common search space 112, in which UEs search for common DCI. The UE-specific search spaces depend on the UE unique identifier and may overlap with common search space, in which UE-specific DCIs can be allocated.
Only the initial positions for search spaces can be known. For common search space, the initial positions are the first CCE of control region. For UE-specific search space, the initial positions can be obtained by a predetermined HASH function of the UE unique identifier. Thus, the search spaces refer to the region in which the UE attempts to decode a number of PDCCH candidates. In other words, each UE shall monitor a set of PDCCH candidates for control information in every non-discontinuous reception (DRX) subframe. Each UE monitoring implies a blind decoding attempt to decode a DCI in the set of PDCCH candidates according to all the monitoring DCI formats. The DCI formats that the UE shall monitor depend on the configured transmission mode. For 3GPP LTE release-8 standard, each UE shall monitor two DCI payload sizes while the transmission mode is given.
TABLE 1Supported PDCCH formatsNumber of CCEsNumber of PDCCHPDCCH Format(Aggregation level)bits0172121442428838576
FIG. 1B is a schematic diagram illustrating PDCCH formats in a single component carrier system. Referring to FIG. 1B, the PDCCH format 0 (labeled as F0) consists of 1 CCE; the PDCCH format 1 (labeled as F1) consists of 2 CCEs; the PDCCH format 2 (labeled as F2) consists of 4 CCEs; the PDCCH format 3 (labeled as F3) consists of 8 CCEs. The PDCCH format 0, 1, 2, 3 can also be referred to aggregation levels 1, 2, 4, 8.
FIG. 1C is a schematic diagram illustrating common search space 112 and corresponding PDCCH candidates in a single component carrier system. Referring to FIG. 1C, for example, the common search space 112 consists of 16 CCEs (numbered from 0 to 15), and any common DCI may be encoded into an aggregation level 4 or an aggregation level 8 according to the 3GPP LTE release-8 standard. Each UE shall monitor a set of PDCCH candidates at each of aggregation levels 4 and 8 in common search space (C-SS). The set of common PDCCH candidates is the same for all UEs. In other words, common search space is independent of UE unique identifier.
The common control region consists of a set of CCEs. For example, there are 16 CCEs (equivalent to 1,152 bits) in the common control region, numbered form 0 to 15. There are 4 PDCCH candidates at aggregation level 4 such as a first candidate 131, a second candidate 132, a third candidate 133, a fourth candidate 134 (shown in FIG. 1C), and 2 PDCCH candidates at aggregation level 8 such as a first candidate 135, a second candidate 136 (shown in FIG. 1C). The initial position starts form CCE 0 at both aggregation levels. The set of PDCCH candidates in common search space is shown in FIG. 1C. The aggregation levels defining the C-SS are listed in Table 2 below. There are total 6 PDCCH candidates in the common search space.
TABLE 2PDCCH candidates monitored by a UE in Common Search SpaceCommon Search SpaceNumber of PDCCHTypeAggregation level (L)Size (in CCEs)candidatesCommon41648162
On the other hand, a UE-specific DCI may be encoded into an aggregation level 1, an aggregation level 2, an aggregation level 4 or an aggregation level 8 according to the 3GPP LTE release-8 standard. Each UE shall monitor a set of PDCCH candidates at aggregation level 1, 2, 4 and 8 in the UE-specific search space (UE-SS). The set of UE-specific PDCCH candidates varies with UE unique identifier and the slot number within a radio frame. In other words, the initial positions of UE-specific search space vary from UE to UE.
FIG. 1D is a schematic diagram illustrating UE-specific search space and its corresponding PDCCH candidates in a single carrier system. There are 6 PDCCH candidates at aggregation level 1 such as a first candidate 141, a second candidate 142, a third candidate 143, a fourth candidate 144, a fifth candidate 145, a sixth candidate 146 (shown in FIG. 1D). There are 6 PDCCH candidates at aggregation level 2 such as a first candidate 151, a second candidate 152, a third candidate 153, a fourth candidate 154, a fifth candidate 155, a sixth candidate 156 (shown in FIG. 1D). There are 2 PDCCH candidates at aggregation level 4 such as a first candidate 161, a second candidate 162. There are 2 PDCCH candidates at aggregation level 8 such as a first candidate 171, a second candidate 172. Therefore, the UE-specific search space consists of a set of 6 CCEs at aggregation level 1, 12 CCEs at aggregation level 2, 8 CCEs at aggregation level 4, and 16 CCEs at aggregation level 8. The set of PDCCH candidates in UE-specific search space is shown in FIG. 1D.
Each UE shall monitor one C-SS at each of aggregation levels 4 and 8 and one UE-SS at each of aggregation levels 1, 2, 4, 8. The C-SS and UE-SS may overlap. The aggregation levels defining the UE-SS are listed in Table 3. There are total 16 (6+6+2+2) PDCCH candidates in the UE-SS. It is noted that, in the conventional approach, the search spaces are defined in Table 2 and Table 3.
TABLE 3PDCCH candidates monitored by a UE in UE-specific SearchSpaceUE-Specific Search SpaceNumber of PDCCHTypeAggregation levelSize (in CCEs)candidatesUE-specific16621264828162
Each UE monitors a set of PDCCH candidates in C-SS and UE-SS. Each monitoring implies an attempt to decode control information in the monitoring set. For 3GPP LTE release-8 standard, each UE shall monitoring two DCI payload sizes (or two possible types of codeword lengths) at each PDCCH candidate. Therefore, the maximum number of blind decoding attempts is 12 (6*2) for C-SS and 32 (16*2) for UE-SS. Then, each UE is required to operate a maximum of 44 (12+32) blind decoding for the control region, as shown in Table 4.
TABLE 4The maximum number of blind decodes in single carrier systemsMaximumnumber of blindAggregationNumber of PDCCHTypedetectionslevelcandidatesCommon12 =44(4 + 2) * 282UE-specific32 =16(6 + 6 + 2 + 2) * 2264282C-SS + UE-SS44 = 12 + 32——
Usually, the higher the aggregation level (at which DCI is encoded to more CCEs), the better protection capability it can be achieved. Therefore, the higher aggregation level further lowers decoding error probability. Lower decoding error probability leads to better DCI decoding performance. However, when one DCI for a UE is encoded into more CCEs, there may be higher DCI blocking probability for other UEs.
For 3GPP LTE release-8 system, all communications are carried out in the single bandwidth system. However, carrier aggregation (CA) technology is approved to support high data rate transmission over wide frequency bandwidth and increase channel capacity in next generation communication systems (e.g., 3GPP LTE-Advanced). CA technology composes multiple component carriers in contiguous or non-contiguous frequency band. For multiple component carriers (multi-CC) systems, cross-component carrier (cross-CC) PDCCH scheduling shall be supported for higher data rate transmission. In multi-CC systems, UE shall monitor separate PDCCHs in different component carriers. Each component carrier carries has its own control region. Thus, the cross-CC scheduling will increase the number of blind decoding attempts.
The set of PDCCH candidates to be monitored is defined in term of search spaces in every non-DRX subframe, where a search space at an aggregation level is defined by the set of PDCCH candidates. The initial positions of UE-SS can be determined by hash function of the slot number within a radio subframe and the RNTI value used for the UE, where the RNTI value used for the UE is UE unique identifier.
Multiple component carriers systems will support new transmission schemes and new DCI fields. Therefore, the existing DCI formats may be appended with new fields for multi-CC systems. The new DCI formats support new features, such as enhanced downlink eight antennas transmission and uplink single user MIMO (SU-MIMO). The payload sizes of new DCI formats may not be aligned with the existing DCI payload size. In other words, the number of DCI payload sizes for a UE to monitor may be more than 2 if the new DCI payload size is not aligned with the existing DCI payload sizes. The maximum number of blind decoding attempts may be increased while the new DCI payload size is not aligned with the existing DCI payload sizes.
The maximum number of blind decoding will also be increased with the number of aggregated downlink component carriers, regardless of the maximum supported bandwidth of the UE. In single bandwidth systems, up to 44 blind decoding attempts are required for the control region. When the number of component carriers is increased by up to five, the number of blind decoding will be increased linearly to 220 (44×5) without introducing any new DCI payload size.
Although CA technology can dramatically increase transmission bandwidth and consequent data transmission rate, the maximum number of blind decoding attempts will also be increased linearly with the number of component carriers and the new DCI payload size. When cross-CC scheduling is enabled, the existing DCI formats will be increased by at least 3-bit carrier indicator field (CIF). Since the DCI payload sizes are getting larger, the coding rate will be getting higher at a fixed aggregation level. The higher coding rate implies the lower performance gain. Therefore, it is a major concern to find an approach to lower DCI blocking probability, keep the number of DCI blind decoding fewer, while maintain good DCI decoding performance.